Add placeholder item handling and related unit test for deletion

Jellyfin.Server.Implementations/Item/BaseItemRepository.cs

@@ -63,6 +63,9 @@ public sealed class BaseItemRepository
     /// </summary>
     public static readonly Guid PlaceholderId = Guid.Parse("00000000-0000-0000-0000-000000000001");
 
+    private const string PlaceholderType = "PLACEHOLDER";
+    private const string PlaceholderName = "This is a placeholder item for UserData that has been detacted from its original item";
+
     /// <summary>
     /// This holds all the types in the running assemblies
     /// so that we can de-serialize properly when we don't have strong types.
@@ -129,6 +132,8 @@ public sealed class BaseItemRepository
                 {
                     var date = (DateTime?)DateTime.UtcNow;
 
+                    await EnsurePlaceholderItemAsync(context, cancellationToken).ConfigureAwait(false);
+
                     var relatedItems = ids.SelectMany(f => TraverseHirachyDown(f, context)).ToArray();
 
                     // Remove any UserData entries for the placeholder item that would conflict with the UserData
@@ -196,6 +201,38 @@ public sealed class BaseItemRepository
         }
     }
 
+    private async Task EnsurePlaceholderItemAsync(JellyfinDbContext context, CancellationToken cancellationToken)
+    {
+        if (await context.BaseItems.AnyAsync(e => e.Id == PlaceholderId, cancellationToken).ConfigureAwait(false))
+        {
+            return;
+        }
+
+        var placeholder = new BaseItemEntity
+        {
+            Id = PlaceholderId,
+            Type = PlaceholderType,
+            Name = PlaceholderName
+        };
+
+        await context.BaseItems.AddAsync(placeholder, cancellationToken).ConfigureAwait(false);
+
+        try
+        {
+            await context.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
+            _logger.LogWarning("The detached UserData placeholder item was missing and has been recreated.");
+        }
+        catch (DbUpdateException)
+        {
+            context.Entry(placeholder).State = EntityState.Detached;
+
+            if (!await context.BaseItems.AnyAsync(e => e.Id == PlaceholderId, cancellationToken).ConfigureAwait(false))
+            {
+                throw;
+            }
+        }
+    }
+
     /// <inheritdoc />
     public void UpdateInheritedValues()
     {